In recent years, various solar cells have been proposed as a photoelectric conversion element which converts light energy into electrical energy. Among them, a dye-sensitized solar cell which is described in “Nature” 1991, 353, p. 737-740 or the like by Gratzel et. al. of Ecole Polytechnique de Lausanne in Switzerland in 1991 is used with low cost for materials and processes to be used, and thus, the practical use thereof is expected as the solar cell with low cost.
In general, the dye-sensitized solar cell is configured with a semiconductor electrode which includes a photoelectric conversion layer which is formed of a semiconductor which has adsorbed a dye on a conductive base material, an opposite electrode which is provided with a catalyst layer on a conductive base material opposite to the semiconductor electrode, and an electrolyte layer which is held between the semiconductor electrode and the opposite electrode.
An electrolyte which is obtained by dissolving an iodine redox couple in an organic solvent is generally used for the electrolyte of the dye-sensitized solar cell. The iodine redox couple has excellent performance in which ion conductivity is high, and a speed of reducing a dye in an oxidation state is fast, while responsiveness on a conductive glass surface or a titanium oxide surface of an acting pole is low.
However, since a sublimation property of iodine is high, element sealing is difficult and it causes a decrease of element durability under high temperature conditions. In addition, since iodine has a high corrosive property with respect to a lot of metals, metals which can be used in an element substrate are limited, and an expensive substrate such as conductive glass is necessary to be used. Further, in a case of an element, particularly with a large area, metal power collecting lines are provided on the substrate for high performance, in many cases, however, as in this case, it is necessary to provide a process of preventing contact of the electrolyte and the power collecting lines for preventing corrosion of the metal power collecting lines, operating steps are complicated, and an effective area of the element is decreased.
Further, the iodine redox couple has a strong absorbing property in a visible light area, and in a case of using a solvent with high viscosity such as ionic liquid, it is necessary to increase a concentration of the iodine redox couple to be sufficiently operated as a solar cell element, and accordingly, light absorption of a dye is disturbed and it becomes a reason for performance degradation. In addition, in a case of emphasizing a colorful property of the solar cell using various dyes, particularly in a case of a blue element, the color of iodine becomes impeditive, and thus it is not suitable in a viewpoint of element design.
As described above, the iodine redox couple has high performance as the redox couple, however, since it also has problems, a redox couple which can be replaced with the iodine redox couple is required, and there have been some investigation into the matter (for example, see NPL 1 to 6 and PTL 1).
In NPL 1 to 3, there is a proposal for using a cobalt complex for a redox couple. The cobalt complex shows the same performance as the iodine redox couple under weak optical conditions, however, since it has a large molecular size, a moving speed of the redox couple is slow, and the performance is degraded by half under pseudo solar light irradiation conditions. In addition, the cobalt complex is expensive compared to the iodine redox couple, and it cannot be said to be used practically.
In NPL 4 and 5, there is a proposal for using (SCN)2/SCN−, and (SeCN)2/SeCN− for a redox couple. (SCN)2/SCN− are operated as the redox couple, however it shows a half or less extent of performance compared to the iodine redox couple. (SeCN)2/SeCN− shows high performance compared thereto, however, has problem in safety, and it cannot be said to be used practically. As redox couples which can be used for the photoelectric conversion element other than the iodine redox couple, Br2/Br−, Fe(CN)64−/Fe(CN)63−, Fe2+/Fe3+, S2−/Sn2−, Se2−/Sen2−, V2+/V3+, quinone/hydroquinone, and the like are used, however, there are problems in performance, stability, safety, and the like, and the performance comparable to the iodine redox couple has not been obtained.
As shown in PTL 1, the applicants have made clear that a sulfide compound is used as a redox couple and a conductive polymer is used as a counter electrode catalyst, to be efficiently operated for the photoelectric conversion element. In addition, NPL 6 shows that high photoelectric conversion performance is achieved by using a sulfide redox couple and an organic solvent as an electrolyte.
However, in a case of using a volatile organic solvent such as acetonitrile or ethylene carbonate shown in NPL 6, as an electrolyte of the dye-sensitized solar cell, enclosure of the electrolyte is difficult and practical element durability is difficult to be obtained. Accordingly, there are many examples of using ionic liquid having significantly low volatile property as the electrolyte solvent, however, since the ionic liquid has higher viscosity than the general volatile organic solvent, as shown in PTL 1, there is a problem in that the element performance is lower than the organic solvent electrolyte. In addition, NPL 6 shows a case of using tetramethylammonium salts of the sulfide compound as a reductant of the redox couple, however, in this case, there are problems in that solubility to the ionic liquid is insufficient and satisfied element performance is not exhibited.
In addition, PTL 1 shows a sulfide compound including a thiadiazole skeleton as the redox couple, however, there are problems in that a disulfide compound which is an oxidant of the redox couple particularly has low solubility to the electrolyte solvent such as the ionic liquid (less than 0.2 M), and at the time of using a solvent with high viscosity and low volatile property such as ionic liquid or the like, element performance is degraded. Further, in a case where the concentration of the redox couple is set to be high, there is a problem in that the stability of the redox itself is degraded.
Therefore, an electrolyte solution which can be replaced with an iodine-based solution which does not include a sublimation property and a light absorption property in a visible light area, and has high solubility to a electrolyte solvent, and stability and high performance in the solvent, is required.